Transmission Swaps & Modifications

Transmission Options for the Nailhead

Created By: Russ Martin at

There are several transmission choices for the Nailhead engines, I will start with the easiest to use. The 64-65-66 401-425 came with an excellent automatic trans, the ST400. They have a round shaped bell housing unlike other GM transmissions. This trans is super strong and the 65-66 version had a two speed (switch pitch)torque convertor. The stall is changed in the convertor with an electric solenoid so you can change the stall speed (RPM will raise gearing the trans down) with a flip of a switch! I like using the smaller 64-67 ST300 convertor in lighter cars for even a higher stall RPM. I have a picture of a wiring diagram posted to wire for best performance. You don't have to wire to your brake light switch (as in my diagram) but helps when using a larger camshaft for a better idle. THE SWITCH PITCH AND STANDARD CONVERTORS AND FRONT PUMPS ARE DIFFERENT so don't mix them up.
The 64 is better than the Dynaflow but not as a good as the 65-66 ST400 (available in 67 also but has the BOP pattern and will not bolt to the Nailhead). The 64 trans case and valve body are one year only so no shift kits are available and the convertor is the standard GM type. This trans can be identified by the single wire connecter (switch pitch has two) at the rear and a weld on shift lever (the 65 and later bolt on). The 64 also is lacking in controlling down shifts, it shift 3 times but only has a low and drive. If you want to hold it in each gear you start off in L, wind it up and shift to D, that puts it in second gear, now pull it back into L and that holds it in second, wind it up again and shift to D and you are in third. Here is the part I don't like, say you are driving in the mountains, the turns are tighter and slower, you drop in down to L holding it in second gear, then you have to slow down to about 25 mph to make a tighter turn and bam! it goes into low, then you faster, put it into D to put it back in second and and back to low to hold it there.WHEW! I drove a 64 Riviera everyday for 13 years so I know. (A customer gave me a solution for this problem, add a toggle switch inside the car that powers the kick down switch and it will hold the trans in gear!) OK, now you want to know if the 64-66 trans will bolt to earlier Nailheads right? you can bolt this trans on to all Nailheads from 57-63 with my special crank adapter bushing and the correct flex plate, the 59-63 can use the 401-425 flexplate but because ALL Buicks are externally balanced and different flexplate balanced to the 364 spec's must be used. The 57-63 starters (yes 4 different ones used) have a 1/2" longer snout for the Dynaflow so the starter from the 64-66 (or my mini gear reduction starter) must be used and will bolt onto the 61-66 blocks. The 57-60 364-401's will need one of special starters with a different mounting bolt pattern. NEW OPTION, we now have starter spacers so you can keep your original 57-63 starter!! Another trans option is the 4l60-E overdrive, bolt on bell housing, clean looks and function for all 364-401-425 engines . We also have adapters to mount other Chevy automatics behind the Nailhead . BEWARE of venders selling UNBALANCED flex plates and flywheels for less money, you will have to pay as much as 150.00 to balance the flywheel so it will cost more! The 53-56 264/322 has very few options, the bell housing and flywheels look the same as the 57-66 but in fact they are about 1" larger. We sell an adapter for the Chevy 350-400-700R4-2004R and it is possible to adapt a Ford C-4 with a flat-o-matic adapter bolted to a flathead Ford trans adapter. A custom convertor will have to be built but it is a very neat and tight package when it is done and looks like like it was made for it.